This invention relates to an alloy for the occlusion of hydrogen, and more particularly to a novel, useful multi-element alloy for the occlusion of hydrogen, which alloy is capable of occluding a large amount of hydrogen in the form of a hydride and also capable of readily and rapidly releasing hydrogen upon application thereto of slight heat.
Recently, hydrogen has come to attain increasing prominence as a promising, new form of energy to take the place of fossil fuels. This is because it is clean, is available in literally inexhaustible supplies, is amenable to transportation and storage and does not disturb the cycle of nature. Hydrogen has heretofore been stored in the form of gaseous hydrogen, liquefied hydrogen or a metallic hydride. The method for storing hydrogen in the form of a metallic hydride is arresting particular attention because it permits safe storage of a given amount of hydrogen in the same space as is required for the storage of hydrogen in the form of liquefied hydrogen. The substance used for the storage of hydrogen in the form of a metallic hydride must satisfy the requirements (1) that the substance should be inexpensive and enjoy abundance of supply, (2) that it should easily be activated and possess a high capacity for occlusion of hydrogen, (3) that it should possess a proper dissociation equilibrium pressure in the neighborhood of room temperature and (4) that it should permit occlusion and release of hydrogen reversibly and at a high velocity. Such transient metals as Ti, Zr, La and Mg which have heretofore been recognized to be capable of forming hydrides are such that their hydrides are unusually stable thermally and do not release hydrogen until their temperatures are elevated beyond the level of 300.degree. C. Therefore, these metals are not feasible for the storage of hydrogen. As materials for hydrogen storage, there have been developed various alloys including Ti-Ni alloy (Nippon Kagaku Kaishi, 1267-1272, 1975), Ti-Co alloy (Nippon Kagaku Kaishi, 1267-1272, 1975), Ti-Fe alloy (J. J. Reilly, R. H. Wiswall, Jr.: Inorganic Chemistry, 13, 218, 1974), La-Ni alloy (J. H. N. VanVucht, F. A. Kuijpers, H. C. A. M. Burning: Philips Research Reports, 25, 133: 1970--U.S. Pat. No. 3,825,418) and Mg-Ni alloy (J. J. Reilly, R. H. Wiswall, Jr.: Inorganic Chemistry, 7, 2254: 1968). However, these alloys are less than perfect as materials for the occlusion of hydrogen.
Of the alloys mentioned above, those containing Ti, La and Mg possess thermal stability similar to the aforementioned metals Ti, La and Mg, require much time for the occlusion and release of hydrogen and do not permit ready activation. The metals as components for the alloys are required to possess extremely high purity, thus posing a problem from the economic point of view. Further, since the alloy's capacities for the occlusion of hydrogen are affected by the purity of hydrogen, the hydrogen itself must be of very high purity. As described above, the materials heretofore suggested for the storage of hydrogen suffer from various disadvantages. No metals or alloys have yet been perfected which satisfy all the requirements indispensable for the purpose of hydrogen occlusion.
The inventors continued a study in search of an alloy capable of satisfying all these requirements. We developed an alloy comprising misch metal (Mm) and nickel or cobalt and possessing a high capacity for occlusion of hydrogen in the form of hydride and filed a patent application covering the alloy composition and the method for its use (Japanese Patent Application No. 52927/1975). The alloy has an advantage that it is inexpensive, permits occlusion and release of hydrogen at a low temperature and occludes hydrogen to a density substantially equal to the density involved in the storage of hydrogen in the form of liquefied hydrogen. Comparison of the misch metal-nickel (MmNi.sub.5) alloy with the misch metal-cobalt (MmCo.sub.5) alloy in terms of the maximum capacity for the occlusion of hydrogen in the form of hydride (1.5% by weight for MmNi.sub.5 and 0.7% by weight for MmCo.sub.5) and the dissociation equilibrium pressure (14 atmospheres for MmNi.sub.5 and 0.8 atmospheres for MmCo.sub.5) reveals that the MmNi.sub.5 hydride possesses a high capacity for hydrogen occlusion and a high dissociation equilibrium pressure at room temperature, whereas the MmCo.sub.5 hydride possesses a low capacity for hydrogen occlusion and a low dissociation equilibrium pressure. Thus, the two alloys are opposite to each other in their properties. As an improvement to this invention, the inventors developed an alloy of the general formula, MmNi.sub.5-x Co.sub.x, as disclosed in U.S. Pat. No. 4,147,536 dated Apr. 3, 1979. This alloy of a molecular formula, MmNi.sub.2.5 CO.sub.2.5, for example, possesses 1.18% by weight of capacity for hydrogen occlusion and 3.3 atmospheres of dissociation equilibrium pressure at 20.degree. C., indicating that it is equal to MmNi.sub.5 in the capacity for hydrogen occlusion and close to MmCo.sub.5 in the dissociation equilibrium pressure. These alloys nevertheless suffer from a disadvantage that their velocities of hydrogen occlusion are low and their other properties have room for further improvement.
A primary object of this invention is to provide a misch metal-based alloy of a new composition for the occlusion of hydrogen, which is inexpensive and capable of readily occluding a large amount of hydrogen at a high velocity to a high density and releasing hydrogen under mild conditions and possesses a proper dissociation equilibrium pressure, thus meeting practical requirements for use in stationary hydrogen-storage facilities.